Long distance optical communication systems, such as used in transoceanic fiber systems require regenerators or amplifiers to regenerate and amplify the information bearing light signal that attenuates as it moves through the optical fiber. Typically, a single-mode fiber system can transmit 2.4 GBit/s of digital information over distances of 30-50 kilometers before the signal's attenuation requires amplification.
Prior art amplification devices include electro-optic regeneration-repeaters which convert an optical signal into electronic form for amplification. An electro-optic regeneration-repeater is similar to an optical receiver and transmitter placed back-to-back so that the receiver output drives the transmitter. Appropriate electronic circuits digitally "clean up" and regenerate the signal.
Although the electro-optic regeneration-repeaters do amplify the signal adequately, they are complex. In a transoceanic fiber optic system, these regeneration-repeaters may be inadequate because their complexity requires greater maintenance than desired. The ocean depths where such systems require a less complex amplification system.
For transoceanic systems, it would be more desirable to use an optical amplifier which is conceptually simpler than an electro-optic regenerator. Optical amplifiers boost the optical signal strength internally without converting the signal into electrical form. Optical amplifiers use fibers doped with rare-earths, and work on stimulated emission principles similar to lasers, but designed to amplify signals from an external source rather than generate their own light.
The most common type of optic amplifier is an erbium-doped amplifier. These erbium-doped amplifiers work in the wavelength region of about 1530 to 1560 nm and use an external pump light signal to stimulate the erbium ions, which give off photon energy when stimulated. It has been found that a pump wavelength of around 530, 670, 800, 980, and 1480 nm is sufficient to provide amplification of as high as 40 dB in many of these optical amplifiers. At the output of the amplifier, there is an amplified signal with the pump power effectively converted into the signal power.
One drawback of an optical amplifier system is the production of an amplified spontaneous emission (ASE). Some of this emission (ASE) will be in the same wavelength band as the signal and behave as noise, causing a signal-to-noise degradation. Any out-of-band ASE can be filtered, but the in-band ASE cannot be filtered.
The in-band ASE-induced noise causes "signal spontaneous beat noise". As the signal progresses from one optical amplifier to the next and then through subsequent amplifiers, the ASE is added together. As the ASE builds up it becomes a component of the total output power from the amplifiers, thus robbing the system of signal power not only from the standpoint of the signal-to-noise ratio, but also from the standpoint of the finite saturated output power available from an amplifier.
In addition to the problem of induced ASE, there are also the amplification problems caused by a second order effect related to the polarization of the light signal. When a linearly polarized light signal is amplified, the gain of the amplifier is saturated slightly more in the signal polarization than orthogonal to it. This effect is referred to as polarization holeburning. The erbium ions located in the glass of the fiber have an anisotropy so that we have a preferential stimulation of ions which have a local field orientation. That subpopulation of erbium ions will see more saturation and provide more of the gain to the signal. Additionally, at the same time, the ASE builds up in the polarization orthogonal to that of the incident light signal.
These drawbacks would not exist if the signal light were circularly polarized so that the signal light automatically sampled all the subpopulation of the erbium ions. The circularly polarized light would insure that there was no gain difference in one axis of the transmission.
Some prior art amplification systems have attempted to scramble the signal polarization at the source. For example, two close wavelengths generated from a laser transmitter could be transmitted to the same modulator of the transmitter. This enables random polarization of the signal, thereby preventing gain anisotropy caused by a linearly-polarized signals. However, such system may impose other design constraints so that the transmitter design is not as effective as desired.